Optical receiver

Abstract

An optical receiver according to the present invention comprises: a photoelectric conversion unit that converts a received optical signal into an FM signal which is an electric signal; and an amplitude correction unit that corrects the amplitude of a carrier wave or sideband of the FM signal on the basis of a modulation index in the modulation of the FM signal.

Description

Optical receiving device

[0001] The present invention relates to an optical receiving device.

[0002] In the wideband RF signal transmission system described in Non-Patent Document 1, a transmitter generates FM signals by FM-modulating signals to be transmitted using a batch FM conversion method. The signals to be transmitted are signals in multiple frequency bands (channels). The transmitter then modulates the FM signals using an intensity modulator, converts them into optical signals, and transmits them to a receiver.

[0003] The receiver receives the FM signal, converts the optical signal into an electrical signal, and then demodulates the electrical signal to obtain the original signal. The operation of the receiver is described in Non-Patent Document 2.

[0004] T. Shimoba et al., "Study on a wideband RF signal transmission system using FM batch conversion method with all-channel phase modulation," IEICE General Conference 2021. T. Shimoba et al., "Optical video distribution technology using FM batch conversion method," IEICE, IEICE Technical Report, CS2019-84, IE2019-64 (2019-12)

[0005] However, distortion occurs in the received signal, and depending on the frequency band, the signal quality may deteriorate after demodulation. Therefore, there is a need to improve the distortion characteristics of the received signal.

[0006] An object of the present invention is to improve the distortion characteristics of a received signal.

[0007] One aspect of the present invention is an optical receiving device comprising an opto-electrical conversion unit that converts a received optical signal into an FM signal, which is an electrical signal, and an amplitude correction unit that corrects the amplitude of a carrier wave or a side wave of the FM signal based on a modulation index in the modulation of the FM signal.

[0008] According to the present invention, the distortion characteristics of the received signal can be improved.

[0009] FIG. 1 is a diagram illustrating a configuration of an optical communication system according to a first embodiment; FIG. 2 is a diagram illustrating an example of a carrier wave and a side wave; FIG. 3 is a diagram illustrating an example of an FM signal before correction; FIG. 4 is a diagram illustrating an example of an FM signal after correction; FIG. 5 is a diagram illustrating an example of the configuration of an amplitude correction unit; FIG. 6 is a flowchart illustrating the operation of an optical receiving device according to the first embodiment; and FIG. 7 is a diagram illustrating an example of the configuration of an optical communication system according to a second embodiment.

[0010] Embodiments of the present invention will be described in detail with reference to the drawings. Fig. 1 is a diagram showing the configuration of an optical communication system 1 according to a first embodiment. The optical communication system 1 is a system for transmitting video by optical signals. The optical communication system 1 includes an optical transmitting device 10, an optical transmission line 20, and an optical receiving device 30.

[0011] The optical transmitter 10 receives input multi-channel video signals from facilities such as a broadcasting station. The multi-channel video signals are frequency-multiplexed signals of video signals with different frequencies for each channel. The video signals for each channel are, for example, CATV signals, BS / CS right-hand circular polarization IF signals, and BS / CS left-hand circular polarization IF signals. The optical transmitter 10 includes an FM modulator 11 and an intensity modulator 12. The FM modulator 11 FM-modulates the input multi-channel video signals to generate FM signals. The intensity modulator 12 intensity-modulates the FM signals generated by the FM modulator 11 and converts them into optical signals. The intensity modulator 12 transmits the optical signals to the optical receiver 30 via an optical transmission path 20.

[0012] An FM signal has a carrier wave and sidewaves. The sidewaves are divided into upper sidewaves, which are waves with frequencies higher than the carrier wave, and lower sidewaves, which are waves with frequencies higher than the carrier wave. The upper sidewaves are further divided into first upper sidewave, second upper sidewave, etc., in order of their frequency proximity to the carrier wave, and the lower sidewaves are divided into first lower sidewave, second lower sidewave, etc., in order of their frequency proximity to the carrier wave. FIG. 2 shows an example of a carrier wave and sidewaves. In FIG. 2, the sidewaves are shown as the first upper sidewave, second upper sidewave, third upper sidewave, fourth upper sidewave, first lower sidewave, second lower sidewave, third lower sidewave, and fourth lower sidewave. The waveform of an FM signal varies depending on the modulation index used in FM modulation. For example, increasing the modulation index tends to spread the amplitude of an FM signal over a wide bandwidth. The sidewaves of an FM signal generally tend to have smaller amplitudes the further away they are from the carrier wave in frequency. However, even with smaller amplitudes, theoretically, sidewaves can occur up to an infinite distance. In other words, FM signals have a radio bandwidth. However, since it is not practical to handle signals with infinite bandwidth, a threshold is generally set for the amplitude, and sidewaves with amplitudes smaller than that threshold are ignored and treated as nonexistent. In this specification, we also treat each FM signal as having a finite bandwidth, assuming that sidewaves below a preset amplitude threshold do not exist.

[0013] Returning to Fig. 1, the optical receiving device 30 will be described. The optical receiving device 30 receives an optical signal from the optical transmitting device 10, and demodulates the optical signal by intensity demodulating and FM demodulating the optical signal to demodulate a multi-channel video signal. The optical receiving device 30 outputs the demodulated multi-channel video signal to, for example, an external video playback device 90. The video playback device 90 is, for example, a television.

[0014] The optical receiving device 30 includes a photoelectric conversion unit 31, an amplitude correction unit 32, and a demodulation unit 33. The photoelectric conversion unit 31 converts an optical signal into an electrical signal. The photoelectric conversion unit 31 includes, for example, a photodiode. The signal generated by conversion in the photoelectric conversion unit 31 is an FM signal. The amplitude correction unit 32 corrects the amplitude of the FM signal generated by the photoelectric conversion unit 31. The demodulation unit 33 demodulates the FM signal whose amplitude has been corrected by the amplitude correction unit 32.

[0015] The operation of the amplitude correction unit 32 will be described in detail below. FIG. 3 is a diagram illustrating an example of an FM signal before correction. Comparing the transmitting FM signal (shown in FIG. 2) generated by FM modulation of a multi-channel video signal by the FM modulation unit 11 in the optical transmitting device 10 with the receiving FM signal (shown in FIG. 3) generated by conversion by the photoelectric conversion unit 31 in the optical receiving device 30, distortion of the transmitted optical signal due to influences such as distortion in the optical transmission path 20 may cause changes in the amplitude of some of the carrier and sidewaves of the receiving FM signal. In FIG. 3, the ideal value of the amplitude of the FM signal is indicated by a dashed line, and the amplitude of the receiving FM signal is indicated by a solid line. In the FM signal shown in FIG. 3, the amplitude of the first lower sidewave is smaller and the amplitude of the second lower sidewave is larger. The amplitude correction unit 32 corrects the magnitude of the amplitude of the carrier and / or predetermined sidewaves. FIG. 4 is a diagram illustrating an example of an FM signal after correction. 4, the FM signal before correction is shown by a broken line, and the FM signal after correction is shown by a solid line. The amplitudes of the first and second lower sidebands, whose amplitudes have been changed, have been corrected.

[0016] The amplitude corrector 32 corrects the magnitude of the amplitude of the carrier wave and / or predetermined sidewaves based on the modulation index of the FM modulation in the optical transmitter 10. The modulation index of the FM modulation in the optical transmitter 10 may be determined in advance and set in the amplitude corrector 32, or may be set in the amplitude corrector 32 by the optical transmitter 10 transmitting the index to the optical receiver 30 via a transmission path separate from the optical transmission path 20.

[0017] The amplitude correction unit 32 includes, for example, a bandpass filter corresponding to the frequencies of the carrier wave and each sidewave. The amplitude correction unit 32 extracts the carrier wave and each sidewave from the FM signal using, for example, the bandpass filter. The amplitude correction unit 32 compares, for example, the amplitude of the extracted carrier wave and each sidewave with the amplitude of the carrier wave and each sidewave determined by the modulation index (values ​​called "true values"). For example, based on the comparison result, if the amplitude of the extracted carrier wave and each sidewave is greater than the true value, the amplitude correction unit 32 attenuates the amplitude of the carrier wave and / or sidewave. If the amplitude of the extracted carrier wave and each sidewave is smaller than the true value, the amplitude correction unit 32 amplifies the amplitude of the carrier wave and / or sidewave. For example, based on the comparison result, the amplitude correction unit 32 corrects the amplitude of the extracted carrier wave and each sidewave so that the amplitude of the extracted carrier wave and each sidewave becomes equal to the true value. If the magnitude of the amplitude of the extracted carrier wave or side wave is A and the true value of the carrier wave or side wave is B, the amplitude correction unit 32 can make the magnitude of the amplitude of the extracted carrier wave or side wave equal to the true value by multiplying the magnitude of the amplitude of the extracted carrier wave or side wave by B / A.

[0018] Here, the amplitude of the extracted carrier or sidewave to be compared with the true value may be the amplitude measured from a single extracted carrier or sidewave. However, it is preferably the average of the amplitudes measured from multiple extracted carriers or sidewaves, and more preferably the time average of the amplitudes measured from the carrier or sidewave. In reality, the modulation signal used to perform FM modulation by the optical transmitter 10 fluctuates and has an inconstant frequency and amplitude. Therefore, the amplitudes of the carrier and sidewave also fluctuate over time and are not constant. As a result, the carrier and sidewave extracted at a given time may be affected by fluctuations in the modulation signal. However, the time average of the amplitudes of the carrier and sidewave is a constant value because the time fluctuations in the amplitude of the sidewave are canceled out. Therefore, the amplitude correction unit 32 can more accurately correct the amplitude of the carrier or sidewave by comparing the time average of the amplitude measured from the carrier or sidewave with the true value of the carrier or sidewave.

[0019] 5 is a diagram showing an example of the configuration of the amplitude correction unit 32. The amplitude correction unit 32 includes a band-pass filter 321, a time-average calculation unit 322, a correction unit 323, and a multiplexer 324. The amplitude correction unit 32 includes N time-average calculation units 322-1 to N and N correction units 323-1 to N (N is the number of carriers and sidewaves processed by the amplitude correction unit 32) for each sidewave or each sidewave frequency.

[0020] The bandpass filter 321 extracts a carrier wave and each sidewave from the input FM signal. The bandpass filter 321 outputs the extracted carrier wave and each sidewave to the corresponding time average calculation unit 322 and correction unit 323. The time average calculation unit 322 measures the time average of the amplitude of the corresponding carrier wave or sidewave. The time average calculation unit 322 measures, for example, the amplitude of the carrier wave or sidewave input from the bandpass filter 321 and accumulates the measurement values. The time average calculation unit 322 can measure the time average of the amplitude of the carrier wave or sidewave by calculating the average of the accumulated measurement values.

[0021] The correction unit 323 compares the time average of the amplitude of the carrier wave or sidewave measured by the time average calculation unit 322 with the true value determined by the modulation index, and determines the correction degree for the amplitude of the carrier wave or sidewave input from the bandpass filter 321. If the time average of the amplitude of the sidewave is A and the true value is B, the correction unit 323 determines the correction degree to be B / A. The correction unit 323 corrects the amplitude of the carrier wave or sidewave input from the bandpass filter 321 to be B / A times the value indicated by the correction degree.

[0022] The multiplexer 324 multiplexes the carrier wave corrected by the corrector 323 and each side wave.

[0023] Although the optical receiving device 30 measures the amplitude of the carrier wave and each sidewave, it may measure values ​​that have a one-to-one correspondence with the amplitude. Alternatively, the optical receiving device 30 may measure the energy or power of the carrier wave and each sidewave.

[0024] 6 is a flowchart showing the operation of the optical receiving device 30 according to the first embodiment. The photoelectric conversion unit 31 converts an optical signal into an electrical signal (step S11). The amplitude correction unit 32 corrects the amplitude of the carrier wave and / or predetermined side waves based on the modulation index of the FM modulation in the optical transmitting device 10 (step S12). The demodulation unit 33 demodulates the FM signal whose amplitude has been corrected by the amplitude correction unit 32 (step S13).

[0025] As described above, the optical receiving device 30 can correct the amplitude of the carrier wave and each sidewave of the FM signal. By correcting the amplitude of the carrier wave and each sidewave of the FM signal, distortion that occurs during transmission of the optical signal can be eliminated, and the quality of the ultimately demodulated multi-channel video signal can be improved.

[0026] Second Embodiment In an optical communication system 1 according to a second embodiment, the optical transmitting device 10 may FM-modulate a multi-channel video signal multiple times, and the optical receiving device 30 may perform FM demodulation processing multiple times on the electrical signals generated by conversion by the photoelectric conversion unit 31. The optical receiving device 30 may perform amplitude correction processing multiple times on the electrical signals generated by conversion by the photoelectric conversion unit 31. The number of times the optical transmitting device 10 performs FM modulation is the same as the number of times the optical receiving device 30 performs FM demodulation. The number of times the optical receiving device 30 performs amplitude correction is equal to or less than the number of times the optical transmitting device 10 performs FM modulation.

[0027] FIG. 7 is a diagram illustrating an example of the configuration of an optical communication system 1 according to a second embodiment. In the optical communication system 1 illustrated in FIG. 7, the optical transmitting device 10 performs FM modulation twice, and the optical receiving device 30 performs amplitude correction twice and FM demodulation twice. Therefore, the optical transmitting device 10 illustrated in FIG. 7 includes two FM modulation units 11-1 and 11-2, and the optical receiving device 30 illustrated in FIG. 7 includes two amplitude correction units 32-1 and 32-2 and two demodulation units 33-1 and 33-2. The optical transmitting device 10 illustrated in FIG. 7 performs FM modulation twice on a multi-channel video signal using the FM modulation unit 11-1 and the FM modulation unit 11-2. The amplitude correction unit 32-1 corrects the amplitude of the signal converted by the photoelectric conversion unit 31. The demodulation unit 33-1 FM demodulates the signal whose amplitude has been corrected by the amplitude correction unit 32-1. The amplitude correction unit 32-2 corrects the amplitude of the signal demodulated by the demodulation unit 33-1. The demodulation unit 33-2 FM demodulates the signal whose amplitude has been corrected by the amplitude correction unit 32-2.

[0028] The demodulation unit 33 has a corresponding FM modulation unit 11 and demodulates the signal modulated by the corresponding FM modulation unit 11. The multi-stage modulation in the optical transmitting device 10 is the reverse process of the multi-stage demodulation in the optical receiving device 30. Therefore, in the example shown in Figure 7, the FM modulation unit 11-1 corresponds to the demodulation unit 33-2, and the FM modulation unit 11-2 corresponds to the demodulation unit 33-1.

[0029] The amplitude correction unit 32 corresponds to the demodulation unit 33 in the same manner. In the example shown in Fig. 7, the FM modulation unit 11-1 corresponds to the amplitude correction unit 32-2, and the FM modulation unit 11-2 corresponds to the amplitude correction unit 32-1. The amplitude correction unit 32 corrects the amplitude of the input signal using the modulation index of the corresponding FM modulation unit 11-1. In the example shown in Fig. 7, the amplitude correction unit 32-1 uses the modulation index of the FM modulation unit 11-2, and the amplitude correction unit 32-2 uses the modulation index of the FM modulation unit 11-1.

[0030] In the first embodiment, the optical receiving device 30 performs amplitude correction only once. At this time, the amplitude is corrected in the optical receiving device 30, but errors other than amplitude in the waveform (for example, errors in phase or frequency) are not corrected. In the second embodiment, the optical receiving device 30 performs amplitude correction multiple times. As an example of the second embodiment, an example in which the optical receiving device 30 shown in FIG. 7 performs amplitude correction and FM demodulation processing twice will be described below. Hereinafter, a signal whose amplitude has been corrected by the amplitude correction unit 32-1 will be referred to as a "first corrected signal," a signal demodulated by the demodulation unit 33-1 will be referred to as a "first demodulated signal," a signal whose amplitude has been corrected by the amplitude correction unit 32-2 will be referred to as a "second corrected signal," and a signal demodulated by the demodulation unit 33-2 will be referred to as a "second demodulated signal."

[0031] As described above, the amplitude is corrected in the first correction signal, but errors other than the amplitude in the waveform remain. Then, a first demodulated signal is generated based on the first correction signal. Because the first demodulated signal is generated based on the first correction signal, errors other than the amplitude in the first correction signal appear as various errors (amplitude error, phase error, frequency error) in the first demodulated signal. The amplitude error appearing in the first demodulated signal is corrected by the second correction signal. This allows the optical receiving device 30 in the second embodiment to correct errors other than the amplitude in the FM signal.

[0032] Although the embodiments of the present invention have been described in detail above with reference to the drawings, the specific configuration is not limited to these embodiments. The above-described embodiments are merely examples, and the present invention can be embodied in various forms with various modifications and improvements based on the knowledge of those skilled in the art, and designs within the scope of the present invention are also included.

[0033] The signal input to the optical transmitter 10 is not limited to a multi-channel video signal, but may be any signal in which signals of different frequencies are frequency-multiplexed.

[0034] REFERENCE SIGNS LIST 1 Optical communication system, 10 Optical transmitter, 20 Optical transmission path, 30 Optical receiver, 31 Photoelectric converter, 32 Amplitude correction unit, 321 Bandpass filter, 322 Time average calculation unit, 323 Correction unit, 324 Multiplexer, 33 Demodulation unit

Claims

1. An optical receiving device comprising: an opto-electrical conversion unit that converts a received optical signal into an FM signal, which is an electrical signal; and an amplitude correction unit that corrects the amplitude of the carrier wave or side wave of the FM signal based on the modulation index in the modulation of the FM signal.

2. The optical receiving device of claim 1, wherein the amplitude correction unit compares the magnitude of the amplitude of the carrier wave and each side wave determined based on the modulation index with the magnitude of the amplitude of the carrier wave and each side wave extracted from the FM signal, and attenuates or amplifies the amplitude of the carrier wave and each side wave extracted from the FM signal so that the magnitude of the amplitude of the carrier wave and each side wave extracted from the FM signal becomes the magnitude of the amplitude of the carrier wave and each side wave determined based on the modulation index.

3. The optical receiving device according to claim 2, wherein the magnitude of the amplitude of the carrier wave and each sidewave extracted from the FM signal is a time average of the magnitude of the amplitude.

4. The optical receiving device according to any one of claims 1 to 3, wherein the amplitude correction unit corrects the amplitude of the carrier wave or sidewave of the FM signal a number of times equal to or less than the number of FM modulations on the transmitting side.

5. An optical receiving device according to any one of claims 1 to 3, wherein the magnitude of the amplitude of the carrier wave and each side wave is such that the magnitude and value of the amplitude of the carrier wave and each side wave correspond one-to-one.

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